The present invention relates generally to multipliers, and more particularly to a four-quadrant multiplier that exhibits lower noise than prior art four-quadrant multipliers.
A method for reducing noise in a four-quadrant multiplier having first and second cross-coupled pairs of differential bipolar transistors, differential input current terminals connected with a first pair of common junctions of the respective pairs of differential transistors and the differential output current terminals cross coupled to form a second pair of common junctions of the respective pairs of differential transistors is described. The method includes providing a noise current path from the differential input current terminals to a bias voltage, the noise current path substantially bypassing the differential output current terminals when the gain of the multiplier is near zero. Preferably, the first pair of junctions are common emitter junctions and the second pair of junctions are common collector junctions, and the noise current path comprises a third pair of transistors having respective emitters connected to the common emitter junctions, having common collectors connected to a bias voltage and common bases connected to a controlling voltage. Overall output noise is substantially reduced, as the current through the differential transistor pairs is shunted instead to the bias voltage, effectively ensuring that no noise current reaches the differential output terminals when the current through the differential transistor pairs is zero, i.e. when the gain of the multiplier is zero.